This invention pertains to a Method and Apparatus for drying or removing moisture from particulate material. In one of its more specific aspects, the present invention relates to circulating the particulate material through heating and cooling zones until the desired dryness or moisture content is achieved. In this specific aspect hot air is caused to move with the particulate material through the heating zone and cool air is caused to move against the flow of the particulate material in the cooling zone. The circulation rate of the particulate material through the dryer and the cooling zone air flow rate can be varied to control the particulate material drying rate and temperature, as well as to control the ratio of moisture removal from the particulate material in the heating and cooling zones.
At present, a number of particulate materials, including most grains, must be dried prior to storage. In the case of grains the final moisture in the dried grain should be controlled and the moisture content should be relatively uniform throughout the grain that is to be stored. Accordingly, a number of drying systems have been devised to dry particulate material.
One of the prior art grain dryers that has been used can be generally referred to as a crossflow dryer. In this dryer particulate material is caused to flow between two perforated walls and hot air is forced through the particulate material in a direction which is approximately perpendicular to the direction of flow of the particulate material. As the hot air passes through the particulate material, the particulate material is dried. In some dryers of this type cool air is passed through the particulate material in the bottom of the dryer and the cool air also travels in a direction which is approximately perpendicular to the direction of flow of the particulate material. In the crossflow dryer the particulate material generally passes through the dryer only once, and it can be difficult to achieve the desired amount of drying in one pass of the particulate material through the dryer and, at the same time maintain an acceptable quality level. Also, the grain temperature in the crossflow dryer approaches the temperature of the drying air and, accordingly, the temperature of the drying air must be limited to prevent excessive particulate material temperatures. Thus, relatively inefficient low temperature air must be used to dry the particulate material in a crossflow type of dryer. Another limitation of the crossflow dryer is that there is very little mixing of the particulate material as it passes through the dryer. The hot air is introduced on one side of the dryer and caused to flow through the particulate material in a direction that is generally perpendicular to the direction of flow of the particulate material. The particulate material is exposed to higher temperature air on one side of the dryer, where the drying air is introduced, and lower temperature air on the other side of the dryer, where the drying air is removed. Therefore, it is very difficult to achieve uniform drying with this type of dryer. It should also be noted that since relatively low temperature drying air is used in this type of dryer that large quantities of air must be forced through the particulate material to achieve the desired dryness.
Another type of prior art grain dryer is the concurrent flow heating-counter flow cooling dryer. In this dryer heated air is introduced into the particulate material in the upper region of the dryer where the heated air flows in the same general direction as the particulate material. The heated air is then exhausted from the particulate material in the central region of the dryer. Cooling air is introduced at the bottom of the dryer and travels in a direction generally counter to the direction of flow of the particulate material. The cool air is also exhausted from the dryer in the central region of the dryer. The heated air and cooling air are normally removed from the dryer at a common exhaust. U.S. Pat. Nos. 3,710,449 and 3,701,203 disclose dryers that can generally be categorized as concurrent flow heating-counter flow cooling dryers. A major disadvantage with the above type of dryer is that the particulate material passes through the dryer only once and, accordingly, the particulate material must be exposed to the hot air for a relatively long period of time to achieve the desired final moisture content. Since the particulate material is exposed to the hot air for a long period of time the particulate material will undergo a significant increase in temperature. Therefore, the temperature of the heated air must be limited to prevent degradation of the particulate material that is being dried. Limiting the temperature of the heated air reduces the potential drying efficiency and also requires larger quantities of air to be forced through the particulate material to achieve the desired amount of drying. In this type of dryer the moisture is removed from the particulate material in one pass through the dryer. Accordingly, the moisture must be removed from the particulate material relatively rapidly as the particulate material passes through the dryer. The relatively rapid removal of the mositure can be detrimental to the physical properties of the particulate material.
According to the invention, there is provided apparatus for drying particulate material comprising an enclosed chamber through which the particulate material is passed. A heating zone in the chamber is adapted for the introduction of a heated fluid therein, the heated fluid is introduced to travel in the same direction as the particulate material through the heating zone to heat and remove moisture from the particulate material. A cooling zone in the chamber is adapted for the introduction of cooling fluid therein, the cooling fluid is introduced to travel in a direction opposite to the direction of travel of the heated particulate material through the cooling zone to cool and remove moisture from the particulate material. Means is provided to recirculate the particulate material through the enclosed chamber until the desired degree of drying is achieved.
There is also provided, according to the invention, a method for drying particulate material comprising passing the particulate material through an enclosed chamber. A heated fluid is introduced into a portion of the enclosed chamber to heat and absorb moisture from the particulate material, the heated fluid travels in the same direction as the particulate material through the enclosed chamber. A cooling fluid is introduced into a portion of the enclosed chamber to cool and absorb moisture from the heated particulate material, the cooling fluid travels in a direction opposite to the direction of travel of the particulate material through the enclosed chamber. The particulate material is recirculated through the enclosed chamber until the desired degree of drying is achieved.
The invention can be used to dry almost any particulate material. The invention is particularly useful, however, in drying grain and other agricultural products. Accordingly, in the description of the invention there will be some reference to specific applications involving agricultural products. However, it should be understood that the invention can be used to dry other particulate material.
It is an object of the invention to provide improved method and apparatus for drying particulate material which will result in the following advantages: (1) improved fuel economy; (2) improved product quality; (3) reduced fire hazard; (4) minimized air pollution; (5) stabilized fuel efficiency over extremely wide drying ranges and operating conditions; (6) improved drying efficiency at lower moisture contents; and (7) achievement of substantially the desired final moisture content in dried particulate material.
It is a further object of the invention to provide improved method and apparatus wherein the particulate material is recirculated through a drying chamber until the desired degree of dryness is achieved.